non / clouseau   0.2.2


Discover java object sizes through questionable sleuthing plus luck.

Scala versions: 2.12 2.11 2.10



                LADY LITTON
        I hope you locate the trouble, monsieur.

                INSPECTER CLOUSEAU
        Madame, it is my business to locate trouble.

Clouseau turns and collides painfully with the doorway.

                INSPECTER CLOUSEAU
        No trouble back there!


Clouseau is a JVM library designed to help estimate the in-memory size of various objects on the JVM. This is done using the machinery provided by java.lang.instrument.Instrumentation as well as the Java reflection API.

Currently you can build the project and use the JAR file it produces in your won projects.

The ultimate goal (not yet realized) is to provide an SBT plugin that makes it easy to launch tests (or a console) from SBT with the instrumentation automatically set up.

Quick Start

Clouseau supports Java 1.6+, and Scala 2.10, 2.11, and 2.12.

Clouseau is available on Maven Central. The easiest way to use Clouseau with SBT is to enable the sbt-javaagent plugin.

In project/plugins.sbt you'd add:

addSbtPlugin("com.lightbend.sbt" % "sbt-javaagent" % "0.1.4")

In build.sbt you'd add:

// snippet assumes you're using scala 2.12.x; change as-needed
javaAgents += "org.spire-math" % "clouseau_2.12" % "0.2.2" % "compile;runtime"

(If you want to use Clouseau in the REPL you'll also need to add the snippet from Using Clouseau in the REPL.)

Less Quick Start

You can also use Clouseau manually using -javaagent explicitly.

Download the Clouseau jar file using the appropriate link:

If you had downloaded Clouseau to path/to/clouseau.jar you'd include Clouseau in your project via the following build.sbt configuration:

// need to use this to set up instrumentation
javaOptions += "-javaagent:path/to/clouseau.jar"

// needed to start a new JVM using the -javaagent
fork := true


The basic API of Clouseau is intended to be very simple to use:

import clouseau.Calculate

val interestingObject = ...

// all sizes represented in bytes
val x: Long = Calculate.sizeOf(interestingObject)
val y: Long = Calculate.staticSizeOf(interestingObject)
val z: Long = Calculate.fullSizeOf(interestingObject)

At a high level, we can separate objects referenced by a particular object into instance members and static members. Static members are defined in a given class and shared by all instances of that class or its subclasses, whereas instance members are not (necessarily) shared.

For example, consider the following:

import clouseau.Calculate._

val x = "this is a sentence"
sizeOf(x)       //  80 bytes
staticSizeOf(x) //  48 bytes
fullSizeOf(x)   // 128 bytes

val o = List(x, x)
sizeOf(o)       // 144 bytes
staticSizeOf(o) //  16 bytes
fullSizeOf(o)   // 216 bytes

First of all, notice that the string x (of 18 characters) takes up 80 bytes, instead of the 19 bytes that a C programmer might expect. Let's put this aside for now. It looks like x also depends on 48 bytes of static data (via java.lang.String), which is shared across all string values in this JVM. In this case, we can add the results of sizeOf(x) and staticSizeOf(x) to get the fullSizeOf(x) -- but be aware that this won't always be the case. (See the Details section for a more in-depth discussion of this.)

Now, looking at o, we see that sizeOf(o) is less than twice sizeOf(x). This is because the list stores two references to x, so we don't count sizeOf(x) twice (although we will count the size of two references). The 144 bytes of sizeOf(o) will include the 80 bytes of x once, as well as 64 bytes of other data. These data are likely references: two different references to x, as well as the references to cons cells that make up a linked list.

The 16 bytes of staticSize(o) includes the static Nil value that all lists share. Notice that fullSize(o) is 216 bytes, which is significantly more than sizeOf(o) + staticSize(o) (160 bytes). The reason here is that we are also including static fields referenced by the values that make up the list (in this case x).

(See the Caveats section to get an idea of the limitations of these kinds of estimates.)

There is also a compatibility API provided for Java. This API exposes static methods that should be easier to call from Java than methods on Scala objects.

import clouseau.compat.Calculate;

Object interestingObject = ...;

// all sizes in bytes
long x = Calculate.sizeOf(interestingObject);
long y = Calculate.staticSizeOf(interestingObject);
long z = Calculate.fullSizeOf(interestingObject);


It can be tricky to differentiate the three top-level methods provided. Here is an overview which defines the methods recursively, and tries to make their relationship to each other clear.

sizeOf(x) is defined as:

  • the sum of sizeOf(_) for all non-static members of x.

staticSizeOf(x) is defined as:

  • the sum of fullSizeOf(_) for all static members of x.

fullSizeOf(x) is defined as:

  • the sum of fullSizeOf(_) for all non-static members of x
  • added to the sum of fullSizeOf(_) for all static members of x.

Some important things to notice are:

  1. We can't assume that any of these counts don't overlap. It's possible that an object counted in sizeOf(x) is also referenced in a static field counted by staticSizeOf(x).

  2. fullSizeOf(x) will potentially include values that weren't counted by either sizeOf or staticSizeOf. For example, if x has a (non-static) field referencing y, and y has a static field referencing z, then z would not be taken into account by sizeOf(x) or staticSizeOf(x), but would be taken into account by fullSizeOf(x).

  3. It is critical that we avoid double-counting, since it's possible to reference the same object multiple times, or to have multiple instances with the same static fields.

Clouseau uses a 64-bit hashing scheme to try to avoid double-counting. We hash objects to avoid double-counting their sizes, and we also hash classes to avoid double-counting their static members. It's possible that hash collisions will cause us to undercount, but in practice this should be very unlikely. See clouseau.Identity.hash for more information.

The lower-level calculate method will return the set of all hash codes that we've seen so far in addition to an estimate. This makes it possible to do more advanced profiling, such as measuring an initial state, followed by one or more measurements which will only measure the additional memory used since the initial sate.

Here's an example of using calculate:

import clouseau.Mode.JustClass
import clouseau.Calculate.{calculate, sizeOf}
import scala.collection.mutable

val s = mutable.Set.empty[Long]

val m0 = (1 to 100) => (i, i.toString)).toMap
val bytes0 = calculate(m0, s, JustClass).bytes

val m1 = m0.updated(99, "ninety-nine")
val bytes1 = calculate(m1, s, JustClass).bytes

println((bytes0, sizeOf(m0))) // (13840,13840)
println((bytes1, sizeOf(m1))) // (336,13856)

The values bytes0 and sizeOf(m0) are identical. This means that all of the data in m0 is being counted for the first time. By contrast, bytes1 is much smaller than sizeOf(m1), which means that most of the objects being referenced by m1 had already been counted by the first calculate call. Only 2.4% of the total size of m1 has to be allocated; the other 97.6% is shared!

(Since s is a mutable set, as long as we use the same set we ensure that repeatedly-referenced objects will not be counted again.)

The Mode used in this example (JustClass) corresponds to the logic of the sizeOf method. The other modes (JustStatic and ClassAndStatic) correspond to the staticSizeOf and fullSizeOf methods respectively.

Clouseau also includes a method for producing human-readable sizes:

import clouseau.Units

(1 to 5).foreach { i =>
  val bytes = math.pow(137, i.toDouble).toLong
// 137B
// 18.3K
// 2.45M
// 336M
// 44.9G

Using Clouseau in the REPL

One natural use of Clouseau is in the REPL, where it can estimate the space used by interactively-constructed values. However SBT does not currently support forking the console command (#1918), making it difficult to use Clouseau interactively. The clouseau-repl module solves this problem by providing a main class which can be run from SBT (i.e. avoiding the console command entirely).

To use clouseau-repl from within your project, first follow the Quick Start guide for including Clouseau. Once Clouseau is included, add the following to your build.sbt file:

// settings from the quick start section...

// add a main class that runs the standard scala REPL in a new process,
// with stdin/stdout set up correctly. (SBT work-around.)
libraryDependencies += "org.spire-math" %% "clouseau-repl" % "0.2.2"
mainClass in Compile := Some("clouseau.Repl")
connectInput in run := true
outputStrategy := Some(StdoutOutput)

After these changes, the run command can be used to launch the REPL.

Hopefully in the future SBT will support running a forked console, at which point this module and configuration will become unnecessary.


Clouseau is based around the getObjectSize method from the java.lang.instrumentation.Instrumentation class. From that method's documentation:

Returns an implementation-specific approximation of the amount of storage consumed by the specified object. The result may include some or all of the object's overhead, and thus is useful for comparison within an implementation but not between implementations. The estimate may change during a single invocation of the JVM.

The values returned from Clouseau are subject to these same provisos.

The project's humorous name is intended to help set expectations (although the project's goal is to be as accurate as possible using the available Java APIs).

Known weaknesses in this version of Clouseau are in its handling of primitive static values (which we can't use getObjectSize to estimate).

If you find results that you believe are incorrect, please open an issue with a minimized test case demonstrating the incorrect result, as well as some analysis (bytecode, profiling, etc.) which shows the correct result along with the JVM version you are using.

Future Work

Here are some directions Clouseau will (hopefully) be moving in:

  1. Improve accuracy of primitives/enumerations.
  2. Verify estimates against other tools (YourKit, JProfiler, etc.)
  3. Compare estimates before/after Hotspot JITs the relevant classes.
  4. Provide better documentation and intuitions around JVM memory usage.
  5. Provide a "fall back" strategy that avoids Instrumentation when unavailable.
  6. Compare different JVMs and JVM versions.
  7. Provide more flexible/extensible API for traversing fields.

See Also

This project was inspired by ObjectExplorer and MemoryMeasurer. The general approach is taken from this project, which unfortunately isn't under active development and doesn't seem to distribute JAR files.

Copyright and License

All code is available to you under the Apache 2 license, available at

Copyright Erik Osheim, 2017-2018.